Porous ceramic honeycomb structures are widely used as catalytic converter substrates and/or diesel particulate filters. The earliest cordierite ceramic honeycombs for these applications consisted of reaction-sintered cordierite substrates prepared from extruded mixtures of talc, alumina, and kaolin such as described in U.S. Pat. No. 3,885,977, for example. These were found to have suitable chemical durability, inertness, refractoriness, and thermal shock resistance for the application, and were generally cost effective to manufacture. For purposes of the present description the term “cordierite” used alone refers to orthorhombic cordierite (orthorhombic Mg2Al4Si5O18 and its solid solutions), indialite (hexagonal Mg2Al4Si5O18 and its solid solutions), and mixtures thereof unless expressly stated otherwise.
The high thermal shock resistance of reaction-sintered cordierite ceramics, which is proportional to the ratio of the modulus of rupture (MOR) to the elastic (Young's) modulus (E) of the ceramic and inversely proportional to the coefficient of thermal expansion (CTE) of the ceramic, is attributed to the development of a specific microstructure during firing that depends on an appropriate choice of raw materials, as well as on the forming and firing methods employed. Desirably, the cordierite crystallites within the ceramic will tend to be preferentially oriented with their negative-expansion z-axes aligned near the plane of the wall, thereby reducing the CTE in the axial and radial directions of the honeycomb.
Further, the development of large (>50 μm) fan-like “domains” of radiating prismatic cordierite crystallites creates internal stresses during cooling that produce microcracks throughout the body. The re-closing of these microcracks during subsequent reheating accommodates some of the expansion of the positive-CTE x- and y-axes of the cordierite crystals, thereby further lowering the overall CTE of the body. The very low CTE which results by virtue of preferred orientation and high degree of microcracking has been considered essential to the thermal durability of the substrates.
The technology developed for catalytic converter substrates is generally adaptable to ceramic diesel particulate filters (DPFs). A monolithic DPF may be prepared from a porous ceramic honeycomb by plugging adjacent channels on alternative ends for example, such as in a checkerboard pattern, to form a “wall-flow” filter with inlet and outlet channels. That structure forces entering exhaust gas to pass through the porous honeycomb walls to traverse the wall structure, thereby acting to collect exhaust particulates, such as soot, on the walls of the inlet channels. Relative to converter substrates, which have a relatively low porosity (25-35%) and fine pore size (about 4 μm median pore size) suitable for the application and adherence of catalyst washcoats, DPFs of the prior art have required a higher porosity and coarser pore size to reduce the back pressure through the wall. This has been achieved by utilizing coarser talc and coarser silica raw materials in conjunction with the addition of a coarse pore former (e.g., graphite powder). The pore former burns out of the walls during firing and contributes to obtaining high porosity.
Recent trends in exhaust after-treatment for both gasoline and diesel engines have placed greater demands on the converters and DPFs. For converters, the shift toward higher cell densities and thinner walls (e.g., 900/2) has created challenges in meeting requirements for strength and erosion resistance. The most recent applications requiring low cell densities and thin walls (e.g., 300/4 or 600/3 configurations (cpsi/twall)) for reduced back pressure offer similar challenges, as does the growing interest in higher porosity converters with lower thermal mass for faster light-off to meet emission standards.
Likewise, whereas 45%-50% porosity has been adequate in the past for DPFs used in retrofit applications, the move toward catalyzed DPFs may require even higher porosities and coarser pore diameters to accommodate the catalyst within the porous walls. Porosities as high as 60%-70% and pore diameters of 20-25 μm are being considered.
All of these trends increase honeycomb strength requirements, yet the need to secure low CTEs in cordierite ceramic honeycombs has continued to dictate that microcracking necessarily be present. Microcracking may place a limit on the maximum strength achievable in the ceramic matrix itself. Thus, changes in the honeycomb cell geometry, such as filleting the wall intersections or providing thicker walls near the periphery of the honeycomb near the skin, have been employed to improve the strength of the body in thin wall designs.
A second problem arising from the use of converters and DPFs comprising microcracked cordierite may be the penetration of very fine catalyst washcoat particles into the microcracks formed in the cordierite matrix. The presence of particles within the microcracks may interfere with closure of the microcracks during heating, essentially pinning the cracks open. This may result in an increase in CTE, and also an increase in elastic modulus, both factors contributing to reduced thermal shock resistance. This may be especially problematic for DPFs, where the catalyst washcoats are normally disposed within the porous walls of the filters.
Finally, the high degree of preferred “planar” orientation of cordierite required for low CTE can also present issues in cases where the degree of preferred orientation between the skin and the cellular matrix is different. This can occur due to a different degree of alignment of the platy raw materials resulting from different shear and flow in the skin and matrix during extrusion through the die. Because of the difference in cordierite orientation, the skin and matrix may have different CTEs. Consequently, during cooling from firing, the skin and matrix can contract at different rates, creating tensile stresses within the skin that may lead to the formation of cracks therein. These cracks may reduce both strength and thermal shock resistance.
Based upon the above discussion, it is apparent that, while microcracking and preferred crystallite orientation have been essential in the past for achieving low CTE combined with high thermal shock resistance in reaction-sintered cordierite ceramics, both of these microstructural features can also create problems that may limit the utility of the material for certain applications. For example, microcracking may result in strength reductions, and may facilitate CTE increases where the microcracks are penetrated by catalysts or particulates. Additionally, variable cordierite domain alignment can result in residual stresses in cordierite honeycombs.